Need help to animate a moving line with onDraw() - JAVA

I made a line via the drawLine method that starts from the bottom and goes to half the screen size. So far so good. Now I wanted to "tilt" the line to the left or right when the screen is pressed. Something like this in the picture:

And when pressed again have it tilt back to the other side and so on. The problem I have now is that when I tilt the line, its "trail" is visible, because I call the drawLine method multiple times.

This is my code:

public class MainGamePanel extends SurfaceView implements SurfaceHolder.Callback {
private static final String TAG = MainGamePanel.class.getSimpleName();

private MainThread thread;
private Paint paint;
private GameLine gameLine;

public MainGamePanel(Context context){
    super(context);

    getHolder().addCallback(this);

    setFocusable(true);
    paint = new Paint();
    paint.setStrokeWidth(50);
    paint.setColor(Color.WHITE);
    gameLine = new GameLine(getWidth()/2,getHeight(),getWidth()/2, getHeight()/2,paint);

    thread = new MainThread(getHolder(), this);
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {}
@Override
public void surfaceCreated(SurfaceHolder holder) {
    thread.setRunning(true);
    thread.start();
    gameLine = new GameLine(getWidth()/2,getHeight(),getWidth()/2, getHeight()/2,paint);

}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
    ...}
@Override
public boolean onTouchEvent(MotionEvent event) {
    if(event.getAction() == MotionEvent.ACTION_DOWN){
        gameLine.setTapped(true);
    }
    return super.onTouchEvent(event);
}

public void render(Canvas canvas) {
    gameLine.drawTheLine(canvas);
}
public void update(){
    gameLine.update();
}}

public class MainThread extends Thread {

private boolean running;
private MainGamePanel gamePanel;
private SurfaceHolder surfaceHolder;

public MainThread(SurfaceHolder surfaceHolder, MainGamePanel gamePanel){
    super();
    this.gamePanel=gamePanel;
    this.surfaceHolder = surfaceHolder;
}

public void setRunning(boolean running){
    this.running = running;
}
@Override
public void run(){
    Canvas canvas;
    while (running){
        canvas = null;
        try {
            canvas = this.surfaceHolder.lockCanvas();
            synchronized (surfaceHolder){
                this.gamePanel.update();
                this.gamePanel.render(canvas);
            }
        }finally {
            if(canvas != null){
                surfaceHolder.unlockCanvasAndPost(canvas);
            }
        }
    }
}}
public class GameLine {

private float startX;
private float startY;
private float stopX;
private float stopY;
private Paint paint;
private boolean tapped = false;


public GameLine(float startX, float startY, float stopX, float stopY, Paint paint) {
    Normal Constructor
}
public boolean getTapped(){
    return tapped;
}
public void setTapped(boolean tapped){
    this.tapped = tapped;
}
public void update(){
    if(tapped){
        this.stopX -=1;
    }
}
public void drawTheLine(Canvas canvas){

    canvas.drawLine(startX, startY,stopX, stopY,paint);
}}

I'm looking for any ideas. Did I attempt this the wrong way? How can I make the trail invisible? I'm grateful for any help. Thank you.

In order to eliminate the trail, you need to throw away the old canvas (to be more precise the old bitmap) and create a new one. In your example i would suggest:

@Override
public void run(){
    Canvas canvas;
    while (running){
        canvas = null;
        try {
            canvas = this.surfaceHolder.lockCanvas();
            synchronized (surfaceHolder){
                this.gamePanel.update();
                canvas = gameLine.drawTheLine();
            }
        }finally {
            if(canvas != null){
                surfaceHolder.unlockCanvasAndPost(canvas);
            }
        }
    }
}

and

public Canvas drawTheLine(){
    Canvas canvas = new Canvas();
    canvas.drawLine(startX, startY,stopX, stopY,paint);
    return canvas;
}